This invention relates to the field of electric circuit protection devices, more particularly to the field of circuit interruption devices, and more particularly still to fusing devices.
Electrically powered devices ranging from industrial motors to television sets, employ fuses to protect their internal circuitry from electric short circuit or overload conditions. The fuse element is typically constructed by connecting a fuse wire between conductive end caps disposed on the ends of an insulative tube. The end caps include a solder coating along their inner surface, to which the ends of the fuse wire are integrally connected. In some cases, the fuse wire may extend through a hole in the end cap and be soldered to the outer surface thereof. Thus, the fuse wire is placed in the tube and is in electrical conductive engagement with the end caps. The fuse is placed in the circuit to be protected such that the fuse link melts when an abnormal overload condition occurs, and the conduction of electricity through that particular circuit should cease.
Devices such as television sets and electric motors must typically carry a surge current, or short term current overload, when starting. This surge current is a function of the electric device, and not the power supply circuit. The surge current state continues until the equipment circuitry reaches an electrical steady state condition, which can take several seconds. The fuse cannot distinguish this condition from a source related overload condition, and will therefore open unless some allowance is made to permit this short term overload to pass through the fuse without melting the fuse wire. To ensure future protection of the equipment, this allowance must not adversely affect the future performance of the fuse. Fuses having the characteristics to perform this duty are known as time lag fuses. One such type of time lag fuse is the spiral wound fuse.
A spiral wound fuse has a tubular insulating body with conductive end caps disposed on each end thereof, and a fuse link assembly soldered to the inner surface of each end cap. It is also known to extend the fuse wire through an opening in the end cap and place the solder connection on the outer surface thereof. The fuse wire assembly includes a core of twisted yarn fibers which are devoid of sizing, and a fuse wire wound around the core in a spiral pattern. The yarn is typically a ceramic material, which is fired in a furnace to remove the sizing placed on the fibers during the manufacturing process. The core-fuse wire combination forms a semi-rigid fuse link assembly which maintains its position when soldered in place inside the tubular body. The use of a plating material such as tin permits the use of a larger cross section fuse wire than would be permissible if it were uncoated. When a circuit overload is encountered, the passage of the excess current through the fuse wire causes the fuse wire to generate heat and thereby elevate the fuse wire temperature. The core acts as a heat sink to draw this heat away from the fuse wire, thereby lowering the fuse wire temperature. The transfer of heat from the fuse wire to the core lengthens the time required before the fuse wire melting temperature is reached, thereby creating a time lag fuse.
To help ensure continued fuse effectiveness for repeated surge current cycles, the fuse wire is constructed of a base metal with its outer surface being plated with another metal such as tin. Under short term surge conditions, the base metal and plated layer remain metallurgically distinct. However, if an overload condition persists, the tin plating material will migrate into the base metal, forming an alloy having a lower melting temperature than the base metal. The longer the duration of the overload, the more migration and attendant alloying which occurs. Ultimately, due to the heat produced by the overload the melting temperature of the alloying fuse wire will be reached, thereby causing the fuse wire to melt and open the fuse. The size of the wire, type of plating material and base metal, and amount of plating may all be modified to change the alloying characteristics of the fuse thereby ensuring that the alloying does not occur until the overload condition persists beyond the expected surge current duration. An example of this type of fuse is depicted in U.S. Pat. No. 4,445,106.
One problem commonly encountered with this type of fuse in the presence of arc quenching fillers is the tendency for the fuse wire and core to metallurgically interact when the fuse opens, leaving a carbon deposit adjacent the fuse wire-core interface. This carbon deposit is electrically conductive, which permits a leakage current to flow through the fuse after the fuse link severs to open the fuse.
The use of a core material has been known for at least fifty years. U.S. Pat. No. 2,157,906, Lohausen, discloses a fuse having a ceramic core. The core appears to be a solid extruded section of ceramic, rather than a twisted bundle of individual fibers.
It is known in the fuse art that the use of silicates packed around the fuse element will help reduce arcing which may occur during fuse opening under short circuit conditions. U.S. Pat. No. 2,007,313, Sherwood, discloses a cartridge fuse having magnesium oxide fillers. These fillers are commonly used to extinguish arcing which occurs at high voltage-high amperage interruptions, which tend to produce significant amounts of heat at a very high rate of generation. It is known that the combination of a heat sink core and silicate fillers to create a time lag fuse with high voltage-high amperage interruption capabilities is impractical because the tin plating on the wire will react with the ceramic core at the temperatures reached during arcing thereby leaving a conductive coating on the core, although the fusing link has severed or partially evaporated. This conductive residue permits continued current flow through the fuse which is unacceptable. Therefore, spiral wound fuses have been limited to low current-low voltage applications where significant arcing is not expected.